The present invention relates to homeotropic alignment liquid crystal films. The homeotropic alignment liquid crystal films of the present invention alone or in combination with other optical films may be used as optical films such as retardation films, viewing angle compensators, elliptical polarizers, and brightness enhancement films. The present invention also relates to image display devices comprising such homeotropic alignment liquid crystal films, such as liquid crystal display devices, organic EL display devices, and PDPs.
Optical films with a refractive index anisotropy have taken industrially important roles as used for enhancing the image quality of a liquid crystal display device. The optical films with a refractive index anisotropy may be broadly classified into those produced by stretching plastic films and those produced by aligning liquid crystalline substances. The latter are more worthy of attention because they have potentials that they achieve structures with various refractive indices.
Films having a larger refractive index in the thickness direction are assumed to be effective in improving the viewing angle of a liquid crystal display device. It is assumed that the use of a homeotropic alignment (vertically aligned) liquid crystal is a close approach to obtain such films. The homeotropic alignment of liquid crystal molecules denotes that the longitudinal axes of liquid crystal molecules are aligned substantially vertically relative to the substrate. It is well known that the homeotropic alignment can be obtained by applying electric field to a pair of glass substrates sandwiching a liquid crystal as done in a liquid crystal display device. However, it is very difficult to form the aligned liquid crystal into a film, and the processes having been reported so far have possessed many problems.
For example, in the processes disclosed in the following Patent Documents 1 to 3, the film is obtained by allowing a main chain polymeric liquid crystalline compound to be homeotropically aligned and fixing the compound by vitrification. It is assumed that in the homeotropic alignment, the molecules of the polymeric compound are aligned in the thickness direction and thus there is a concern that cracking is likely to occur in the plane direction. However, in these processes, no technical measure of for example strengthening the materials by cross linking is taken. In the process disclosed in Patent Document 4, the homeotropic alignment of a side chain polymeric liquid crystalline compound is fixed by vitrification. However, there is a concern in terms of strength that is more serious than where a main chain polymeric liquid crystalline compound is used. In the processes disclosed in Patent Documents 5 and 6 below, a polymerizable low molecular weight liquid crystalline compound is added to a side chain polymeric liquid crystalline compound, but there is a limit in reinforcement thereof because the low molecular weight liquid crystalline compound is solely polymerized.
In the process disclosed in Patent Document 7 below, a material is used wherein a radically polymerizable group or a cationically polymerizable group such as vinyl ether and epoxy groups is introduced into a side chain polymeric liquid crystalline compound. However, in general, radical polymerization undergoes oxygen inhibition and may proceed insufficiently, leading the necessity of large facilities or apparatus for removal of oxygen. The vinyl ether or epoxy group is advantageous in this regard because it does not encounter oxygen inhibition. However, there is a concern that the ether bond of the vinyl ether group is unstable and tends to cleave. It is difficult to introduce the epoxy group into a liquid crystalline material and obtain a high polymerization degree when cross linking is carried out. Furthermore, since a large amount of non-liquid crystalline structural units is introduced into the liquid crystalline material in order to obtain the homeotropic alignment, there still remains a concern regarding the stable exhibition of liquid crystallinity. As described above, there have been remains problems in the conventional production of a homeotropic alignment liquid crystal film.
An image display device such as a liquid crystal display device varies in contrast associated with a change in viewing angle due to the birefringence of the liquid crystal or the like. For the purpose of preventing such contrast variations, a technique has been proposed wherein a retardation film is arranged on the liquid crystal cell of a liquid crystal device so as to compensate the optical characteristics relating to birefringence thereby improving the viewing angle characteristics. A uniaxial or biaxial stretched film is used as such a retardation film for compensation. However, such a stretched film does not necessarily have viewing angle characteristics satisfactory to all liquid crystal cells.
Patent Document 8 discloses a continuously carried-out process for producing a retardation film, characterized in that on one or both surfaces of an elongate thermoplastic resin film is bonded one or more heat-contractive films, and the elongate film is held with the grips of a tenter and contracted in the width direction at Magnification A which is in the range of 0.7 or more to less than 1.0, by allowing the contraction force of the heat-contraction films to be acted, followed by stretching and widening the elongate film at a stretch ratio (%) meeting the requirement that the percentage is equal to or less than that represented by (100-Magnification A×100)×0.15 where the film width excluding the parts held by the grips after the contraction is 100.
In this process, the film is also stretched in the thickness direction, resulting in a retardation film having a retardation in the thickness direction. However, when the main refractive indices in the resulting retardation film plane and the refractive index in the retardation film thickness direction are nx and ny, and nz, respectively and nx>ny, Nz defined by Nz=(nx−nz)/(nx−ny) will be −1.0<Nz<0.1. Therefore, there is a limit in stretching in the thickness direction, and thus the retardation in the thickness direction can not be controlled in a wide range. Furthermore, since in this process, the elongate film is stretched in the thickness direction by heat contraction, the resulting retardation film will be thicker than the elongate film. That is, the retardation film produced by this process has a thickness of 50 to 100 μm, which is not thin enough to meet the low profiling required in a liquid crystal display device or the like.
In the process disclosed in Patent Document 9, a retardation film is produced which a homeotropic alignment liquid crystal film and a stretched film with a retardation function are integrally laminated. The process for producing the homeotropic alignment liquid crystal film is the same as that disclosed in Patent Document 7 and is insufficient because the conventional processes including this process still have problems.
In the vertical alignment mode, which is one of the display modes of a liquid crystal display device, the liquid crystal molecules are aligned vertically to the substrate when no electric voltage is applied thereto and produces a black image when linear polarizers are arranged in a crossing relation to each other on both sides of the liquid crystal cell.
The optical characteristics in the liquid crystal cell is isotropic in the plane direction, and thus an ideal viewing angle compensation therefor can be easily achieved. When an optical element with a negative uniaxial optical anisotropy in the thickness direction of the liquid crystal cell is inserted between one or both surfaces thereof and the linear polarizers in order to compensate the positive uniaxial optical anisotropy in the liquid crystal cell thickness direction, very excellent black image viewing angle characteristics can be obtained.
Upon application of an electric voltage, the liquid crystal molecules changes their alignment from the vertical direction relative to the substrate surface toward the parallel direction. Thereupon, it is difficult to make the liquid crystal alignment uniform. The use of a usual alignment treatment, i.e., a rubbing treatment on the substrate results in a significant deterioration in display quality.
There are proposals for making the liquid crystal alignment uniform upon application of an electric voltage that a uniform alignment is obtained by modifying the shape of the electrodes on the substrates so that an oblique electric field is generated in the liquid crystal layer. Although this method enables the liquid crystal alignment to be uniform, an uneven alignment region is produced when viewed at the micro level and will be a dark region upon application of an electric voltage. Therefore, the transmissivity of the liquid crystal display device will be diminished.
Patent Document 11 proposes a configuration wherein the linear polarizers arranged on the both surfaces of a liquid crystal cell having a liquid crystal layer which may be in a random alignment are replaced by circular polarizers. Replacement of the linear polarizers by circular polarizers each of which is a combination of a linear polarizer and a ¼ wavelength plate can eliminate the dark region produced upon application of an electric voltage and accomplish to produce a liquid crystal display device with high transmissivity. However, the vertical alignment type liquid crystal display device with the circular polarizers has a problem that it has narrower viewing angle characteristics than that with the linear polarizers.
Patent Document 12 proposes an optical anisotropic element with a negative uniaxial optical anisotropy or a biaxial optical anisotropic material for compensating the viewing angle of a vertical alignment type liquid crystal display device with circular polarizers. However, although the optical anisotropic element with a negative uniaxial optical anisotropy can compensate the positive uniaxial optical anisotropy in the cell thickness direction, it fails to compensate the viewing angle characteristics of the ¼ wavelength plate, resulting in insufficient viewing angle characteristics.
Patent Document 1: Japanese Patent Publication No. 2853064
Patent Document 2: Japanese Patent Publication No. 3018120
Patent Document 3: Japanese Patent Publication No. 3078948
Patent Document 4: Japanese Patent Laid-Open Publication No. 2002-174725
Patent Document 5: Japanese Patent Laid-Open Publication No. 2002-333524
Patent Document 6: Japanese Patent Laid-Open Publication No. 2002-333642
Patent Document 7: Japanese Patent Laid-Open Publication No. 2003-2927
Patent Document 8: Japanese Patent Laid-Open Publication No. 2002-304924
Patent Document 9: Japanese Patent Laid-Open Publication No. 2003-149441
Patent Document 10: Japanese Patent Laid-Open Publication No. 2003-2927
Patent Document 11: Japanese Patent Laid-Open Publication No. 2002-40428
Patent Document 12: Japanese Patent Laid-Open Publication No. 2003-207782